1. Field of the Invention
The present invention relates to an automatic voltage-regulating system for an engine generator, comprising an output voltahe detecting means for extracting a single-phase AC output voltage generated in an output winding wound around a stator to output a detection voltage from a detection output terminal, and a switching means for regularizing the single-phase AC output voltage to be substantially at a constant level by selectively permotting conducting and cutting off of a field current supplied from an exciting winding wound around the stator to a field winding wound around a rotor depending upon whether the detection voltage provided by the output voltage detecting means is higher or lower than a predetermined value.
2. Description of the Related Art
A prior art automatic voltage-regulating system for an engine generator is designed such that the field current flowing in a field winding 3 is controlled based on a voltage generated in an exciting winding 4 by turning on and off a switching means 10 including a plurality of transistors 22, 23 and 24 connected through the Darlington circuit by a DC detecting voltage provided by extracting a portion of the voltage generated in output windings 1 and 2, as shown in FIG. 3, thereby regularizing the voltage generated in the output windings 1 and 2. However, if a load 6 connected to either one of the output windings 1 and 2, e.g., to the output winding 1 is a condensive load such as a mercury-arc lamp, a metal halide lamp and the like, there is a possibility that the fracture of the smoothing condenser 8 may occur based on an induced voltage generated in the field winding 3.
More specifically, if the load 6 is a condensive load, an alternating voltage of even-number order higher harmonics, which is not generated when the load 6 is a resistance load, is induced in the field winding 3 wound oround a rotor by flowing of a condensive current in the output winding 1. If one cycle of an alternating voltage of a low-order higher harmonic is considered, electric current, in a minus-side half-wave, flows from a negative electrode 3m of the field winding 3 via a diode 12 to a positive electrode 3p of the field winding 3, as shown by broken lines, and on the other hand, in a plus-side half-wave, electric current flows from the positive electrode 3p of the field winding 3 via the smoothing condenser 8 and a diode 13 to the negative electrode 3m of the field winding 3, as shown by solid lines. Therefore, the smoothing condenser 8 is charged with an induced voltage produced in the field winding 3, and this is repeated through the passage of time, so that a voltage is gradually accumulated in the smoothing condenser 8 and this may finally result in fracturing of the smoothing condenser 8.
To solve the above problem, an automatic voltage-regulating system disclosed in Japanese Utility Model Laid-open No.4-101300 is designed such that the fracture of the smoothing condenser 8 is prevented by the following two circuits: a short circuit which is connected in parallel to the field winding 3 to short-circuit an induced voltage produced in the field winding 3, when the induced voltage exceeds a predetermined value, and an inhibiting circuit comprising a diode connected in series to the field winding 3 in a direction of the flowing of the field current.
In the automatic voltage-regulating system disclosed in the above Japanese Utility Model Laid-open No. 4-101300, however, the short circuit including a thyristor and the inhibiting circuit including the diode are required. Thus, it is impossible to mention that number of parts is small as a circuit arrangement for preventing the fracture of the smoothing condenser in the case the condensive load is connected to the output winding, or to mention that the circuit arrangement is simple.
Accordingly, it is an object of the present invention to provide an automatic voltage-regulating system of the above-described type for an engine generator, wherein the fracture of the smoothing condenser can be prevented despite provision of the condensive load, by additionally providing a simple circuit constructed of a reduced number of parts, thereby providing a stable output voltage.
To achieve the above object, according to the present invention, there is provided an automatic voltage-regulating system for an engine generator, comprising an output voltage detecting means for extracting a single-phase AC output voltage generated in an output winding wound around a stator to output a detection voltage from a detection output terminal, and a switching means for regularizing said single-phase AC output voltage to be substantially at a constant level by selectively permitting conducting and cutting off of a field current supplied from an exciting winding wound around said stator to a field winding wound around a rotor depending upon whether the detection voltage provided by said output voltage detecting means is higher or lower than a predetermined value, wherein the system further comprises a rectifying circuit for rectifying a field current supplied from said exciting winding to said field winding, a smoothing condenser for smoothing a rectified output from said rectifying circuit, a pair of voltage dividing resistors for dividing a charged voltage in said smoothing condenser, and a switch means for forcibly lowering a detection voltage appearing at said detection output terminal of said output voltage detecting means in response to a voltage at a connection point between said pair of voltage dividing resistors exceeding said predetermined value, and wherein when said smoothing condenser is in a state charged with an excessive voltage, said switch means is forcibly turned on in response to said detection voltage being forcibly lowered by said switch means to thereby cause a charged current in said smoothing condenser to be discharged.
With such arrangement, if an induced voltage is produced in the field winding in response to flowing of condensive current in the output winding due to the connection of a condensive load, the smoothing condenser is charged. If the charged voltage in the smoothing condenser becomes high and the voltage at the connection point between both the voltage dividing resistors exceeds the predetermined voltage, a detection voltage appearing at the detection output terminal of the output voltage detecting means is forcibly lowered by the switch means, whereby the switching means is turned on and the charged voltage in the smoothing condenser is discharged to the field winding. Thus, it is possible to prevent the smoothing condenser and the like from being fractured due to the excessive voltage. Moreover, to prevent the fracture of the smoothing condenser, it is only required to add, to the automatic voltage regulating system, a circuit which is simple and constructed of reduced number of parts including a pair of voltage dividing resistors and a switch means.
According to the invention, in addition to the above-mentioned first feature, such a second feature is proposed that said switch means is a transistor which forcibly short-circuits the detection voltage appearing at said detection output terminal by conducting. Owing to this feature, the circuit arrangement for preventing the fracture of the smoothing condenser is further simplified.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.